The invention applies, for example, to a hydraulic motor having two main ducts, i.e. a feed duct and an exhaust duct.
It also applies to motors having three main ducts, such as, for example, those which are described in FR 2 570 157 and in FR 2 685 263. In those motors, the two cylinder capacities behave like two elementary motors each having a first and a second elementary duct serving as either the feed or the exhaust duct. For the two elementary motors, the first elementary duct can be shared, thus forming a first main duct of the overall motor, while the second elementary ducts are separate, thus forming the second and third main ducts of the overall motor.
The device of the invention can be fitted to a motor having two active operating cylinder capacities. The invention is also applicable to hydraulic motors having a plurality of cylinder-capacity selector slides, such as motors having three distinct non-zero cylinder capacities and two cylinder-capacity selector slides, e.g. of the type described in FR 2 611 816.
For example, the motor comprises three groups of distribution ducts, the distribution ducts of the first group being permanently connected to a first main duct, while the distribution ducts of the third group are permanently connected to the second main duct. The three communication ports which open out into the bore of the selector device are respectively designed to be permanently connected to each of the three groups of distribution ducts. Thus, in the first stable position of the slide, the second and third groups of distribution ducts are connected to each other, while the first group of said ducts is isolated from the others. In the second stable position, it is the first and second groups of distribution ducts that are connected to each other, while the third is isolated therefrom.
For example, in the preferred direction of operation of such a hydraulic motor fitted with the cylindercapacity selector device (the forward direction for a vehicle driven by the motor), the first main duct to which the distribution ducts of the first group are connected is a fluid exhaust duct, while the second main duct to which the distribution ducts of the third group are connected is a fluid feed duct. Thus, at large cylinder capacity, the distribution ducts of the first group serve for fluid exhaust while both the distribution ducts of the third group and the distribution ducts of the second group serve for fluid feed.
In the same direction of operation, at the small cylinder capacity, only the distribution ducts of the third group are used for feeding fluid, while the distribution ducts of the second group are connected to the ducts of the first group and are therefore at discharge pressure, such that the ducts of the second group are not fed with fluid at high pressure and do not contribute to delivering driving torque. Thus, the portion of the cylinder capacity that corresponds to the distribution ducts of the second group is inactive.
In the other direction of operation of the motor, and at small cylinder capacity, only the distribution ducts of the third group are connected to the discharge, while the distribution ducts of the first and second groups are connected to the feed. Thus, for example in a hydraulic motor having pistons, the pistons which are connected successively to a duct of the second group and to a duct of the first group tend to remain, between those two successive connections, in their extended position and they no longer contribute to providing driving torque. Nevertheless, these pistons tend to develop a certain amount of resistance to driving torque.
When the selector changes between its two stable positions, the temporary situation in which the passages between the first and second ports and between the second and third ports are simultaneously open serves to prevent the motor jamming during displacement of the selector slide. In this temporary position, all of the distribution ducts are in communication with one another and the motor freewheels.
At constant flow rate from the pump feeding the main feed duct, it is known that in the large cylindercapacity position, the motor develops high torque at relatively slow speed while at small cylinder capacity, the torque developed is lower and the speed is higher.
In the preferred operating direction of a motor, when the selector slide is moved from its second stable position for small cylinder capacity towards its first stable position for large cylinder capacity, there is a changeover between a situation in which the fluid flow delivered by the pump passes from feeding only the distribution ducts of the third group to a situation in which the same fluid flow needs to feed the distribution ducts of both the second and the third groups. Consequently, the fluid pressure in the main feed duct tends to drop, and it is maintained at a value that prevents cavitation phenomena by fluid being delivered from a booster pump in addition to the fluid being delivered by the main pump. This booster delivery of fluid is associated with the fact that the speed of the motor continues to be fast under the effect of the inertia of the load driven by the motor.
At that moment, the fluid flow exhausted by the main exhaust duct suddenly becomes greater than the fluid flow it was previously exhausting since it is now connected not only to the fluid flow delivered by the main pump, but also to the fluid flow delivered by the booster pump.
The pressure in the exhaust duct therefore increases suddenly and the load driven by the motor is braked suddenly. This braking is sufficiently abrupt to generate quite violent jarring.
When the motor is used to drive a vehicle, and when the vehicle has no special equipment, the driver of the vehicle can forestall this jarring only by causing the main pump to deliver a momentary increase in fluid flow rate while simultaneously causing a changeover to take place from small cylinder capacity to large cylinder capacity, assuming that the feed circuit constitutes a closed circuit.
There also exist sophisticated systems for avoiding this jarring effect, that rely on servo-controlling the pressure delivered by the pump and the speed of the motor. Such systems suffer from the drawback of being expensive.
Conversely, when the selector slide is moved from its first stable position for large cylinder capacity to its second stable position for small cylinder capacity, quite a sudden acceleration effect occurs. Although the jarring involved is smaller than in the first case since on changing over to small cylinder capacity, driving torque is also reduced, it is perceived as a fault by the driver of the vehicle.
The invention also applies to a motor having two distinct operating cylinder capacities, and comprising four groups of distribution ducts. In the large cylinder capacity, the groups are connected together in pairs so as to be put into communication respectively with a main feed duct and a main exhaust duct. In the small cylinder capacity, one group is connected to the feed duct, and another group to the exhaust duct, and the two groups corresponding to the deactivated cylinder capacity are connected to each other, e.g. by being put into communication with an auxiliary duct such as a booster duct.
In this case also, the sudden changeover from one cylinder capacity to another causes jarring which can be disagreeable, or even dangerous.
The motor can also have a larger number of cylinder capacities, e.g. three distinct cylinder capacities, i.e. a large, a medium, or a small cylinder capacity. In this case also, it is advisable to avoid sudden jarring when changing over from one cylinder capacity to another.